흔한 동시성 버그

소개

경쟁 상태

코드: 경쟁 상태

public class Main {
  private int count = 0;
  public void increment() { count++; }
  public static void main(String[] args) throws Exception {
    Main obj = new Main();
    Thread t1 = new Thread(() -> { for(int i=0;i<1000;i++) obj.increment(); });
    Thread t2 = new Thread(() -> { for(int i=0;i<1000;i++) obj.increment(); });
    t1.start(); t2.start();
    t1.join(); t2.join();
    System.out.println("Count (expected 2000) = " + obj.count);
  }
}

교착 상태

코드: 교착 상태

public class Main {
  private final Object lock1 = new Object();
  private final Object lock2 = new Object();
  public void method1() {
    synchronized(lock1) {
      try { Thread.sleep(50); } catch(Exception e) {}
      synchronized(lock2) { System.out.println("method1"); }
    }
  }
  public void method2() {
    synchronized(lock2) {
      try { Thread.sleep(50); } catch(Exception e) {}
      synchronized(lock1) { System.out.println("method2"); }
    }
  }
  public static void main(String[] args) {
    Main obj = new Main();
    new Thread(obj::method1).start();
    new Thread(obj::method2).start();
  }
}

코드: 가시성 문제

public class Main {
  private static boolean flag = false;
  public static void main(String[] args) throws Exception {
    Thread writer = new Thread(() -> { flag = true; });
    Thread reader = new Thread(() -> { while(!flag) {} System.out.println("Flag seen true"); });
    writer.start(); reader.start();
    writer.join(); reader.join();
  }
}

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